Steam-cooled gas turbine bucker for reduced tip leakage loss

ABSTRACT

A bucket for a steam turbine includes an airfoil portion having a radially outer tip, the radially outer tip having a thermal barrier coating applied thereto, and wherein the thermal barrier coating is resurfaced to form at least one ridge along the radially outer tip.

BACKGROUND OF THE INVENTION

This invention relates to steam turbine buckets generally and to theincorporation of a tip leakage loss reduction feature in the thermalbarrier coating applied to the bucket tip.

The radially outer tips of gas turbine buckets serve in a hostileenvironment of both high temperature and high rotationally-inducedstress. The life of parts subjected to these conditions is typicallylimited by low-cycle fatigue (LCF) and creep considerations. Inaccordance with conventional practice, a tip cap is welded to the bucketas part of a current manufacturing process for hot gas path sealingpurposes. The addition of a conventional metal seal to the existing tipcap increases the thermal gradient at the tip, however, and thereforedegrades the LCF and creep life. In prior art buckets, this is overcomeby employing film cooling in the bucket tip region. In closed-loopsteam-cooled turbine bucket applications, however, airfoil film coolingcannot be practically applied, as there is only a single closed coolingcircuit. A shroud covering the tip gap and cantilevered across theblade-to-blade gap, as typically applied on stage 2 and stage 3 buckets,is likewise not practical in the first stage due to LCF and creepconsiderations.

Air-cooled buckets typically have a metallic “squealer tip” feature;however, this approach is cast into the bucket which is not feasible forsteam-cooled buckets. Thus, current closed-loop steam-cooled stage 1buckets have no feature to impede fluid flow into the tip gap. As aresult, leakage flow rolls into a vortex, causing a reduction in turbineefficiency by two means. First, the tip flow generates no lift, andcontributes no power-producing torque on the turbine rotor. Second, thetip vortex mixes out with the surrounding flow downstream of the bucket,generating mixing loss.

BRIEF DESCRIPTION OF THE INVENTION

This invention, in one exemplary embodiment, seeks to provide variousgeometry features on the tip cap to impede tip leakage loss withoutdegrading LCF and creep life of a closed-loop steam-cooled bucket.

In the exemplary embodiment, the thickness of a thermal barrier coating(TBC) material applied to the bucket tips (references to the “tips”include the welded-on tip cap unless otherwise noted) is increasedsufficiently to allow a cavity to be machined or ground into the TBCcoating in the bucket tip center portion, along the main camber line ofthe tip. The cavity therefore also defines a ridge about the perimeterof the bucket (at the edge or offset from the edge), along both thesuction and pressure surfaces, similar to a conventional squealer tip. Aridge formed along only the pressure side, or only the suction side ofthe airfoil is also contemplated. In still another variation, a singleridge may be formed along the mean camber line of the TBC-coated buckettip for the purpose of effectively reducing the tip gap over a rotatingunshrouded bucket.

By machining or grinding (or otherwise resurfacing by any suitablemeans) these or similar geometries into the thermal barrier coatingapplied to the bucket tip, the flow of fluid in the gas path from thepressure surface to the suction surface through the tip gap between therotating bucket and the stationary shroud over the bucket is impeded.The thermal barrier coating also reduces the heat flux into the buckettip base metal. The reduction in heat flux will reduce the thermalgradient through the base metal of the tip. This reduction in thermalgradient significantly enhances the LCF and creep life of the buckettip.

Accordingly, the present invention relates to a bucket for a steamturbine comprising an airfoil portion having a radially outer tip, theradially outer tip having a thermal barrier coating applied thereto, andwherein the thermal barrier coating is resurfaced to form at least oneridge along the radially outer tip.

In another aspect, the present invention relates to a bucket for a steamturbine comprising an airfoil portion having a radially outer tip, theradially outer tip having a thermal barrier coating applied thereto, andwherein a cavity is formed in a center portion of the thermal barriercoating along the radially outer tip.

In still another aspect, the present invention relates to a method ofreducing tip leakage loss at a radially outer tip of a turbine bucketcomprising: (a) coating the radially outer tip of the bucket with athermal barrier coating; (b) resurfacing the thermal barrier coating toinclude at least one tip leakage loss feature in the coating, extendingsubstantially the entire length of the tip.

The invention will now be described in detail in connection with thedrawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view of a known closed circuit,steam-cooled turbine bucket;

FIG. 2 is a section taken along the line 2-2 of FIG. 1 but with a tipleakage loss feature formed in the bucket tip cap coating;

FIG. 3 is a section similar to FIG. 2 but illustrating a secondexemplary embodiment of the invention;

FIG. 4 is a section similar to FIG. 2 but illustrating a third exemplaryembodiment of the invention; and

FIG. 5 is a section similar to FIG. 2 but illustrating a fourthexemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional closed-circuit, steam-cooled bucketfor a steam turbine first stage. The bucket 10 is formed with an airfoilportion 12 including a pressure surface (or side) 14 and a suctionsurface (or side) 16. The radially outer tip 18 of the bucket is closedby a tip cap 20 that is welded in place and subsequently sprayed with anotherwise conventional thermal barrier coating (TBC) 22 (FIG. 2).Platform and mounting (e.g., dovetail) portions (not shown) of thebucket are otherwise conventional and need not be described.

In the example portrayed in FIG. 2, the thermal barrier coating 22 isincreased in thickness to T₁ in order to provide sufficient coatingmaterial to accommodate a tip leakage loss-reduction feature asexplained below. More specifically, in the FIG. 2 example, the coating22 is machined to reduce the overall thickness of the coating and toform a cavity 24 in the center region of the tip cap, running along themean camber line of the bucket tip, substantially the entire length ofthe tip. The cross-hatched coating represents the finished, machined orground configuration, while the coating material 30 above thecross-hatched portion is removed. Cavity 24 thus creates ridges 26, 28that extend along the pressure and suction surfaces 14, 16,respectively, and about the perimeter of the bucket tip, but offsetinwardly from the 90° C. tip cap edge. In the exemplary embodiment, theminimum TBC coating thickness at the center of cavity 24 may be on theorder of 30 mils, while the thickness at the ridges 26, 28 may be up toabout 60 mils, and the depth of the cavity 24 may be between about 30and 6 mils. T₁ may be from about 60 to about 110 mils. It will beappreciated that the exact coating thicknesses at the various locationson the tip cap will vary depending on bucket size, tip clearancerequirements and the like. This bucket tip surface feature impedestip-leakage loss without degrading the LCF and creep life of the bucket.

It will be appreciated that other bucket tip surface features are withinthe scope of this invention. For example, in FIG. 3, the cavity 32 isdefined by a smoothly curved surface 34 extending continuously from thesuction side 36 to the pressure side 38 of the airfoil 40, forming aridge 41 about the edge of the tip. Alternatively, as shown in FIG. 4, amachined ridge 42 could be formed in the TBC coating along only thesuction side 44 of the airfoil, or along only the pressure side of theairfoil (not shown), by simply eliminating one side of the cavity.

FIG. 5 illustrates another surface feature in the form of a ridge or rib48 machined or ground into the coating along the mean camber line,equidistantly spaced from the suction side 50 and pressure side 52 ofthe airfoil 54. In fact, any surface feature machined into theTBC-coated bucket tip for the purpose of effectively reducing the tipgap over a rotating unshrouded bucket is contemplated. It is also to beunderstood that the incorporation of various geometries on the tip capcoating is not necessarily limited to buckets with closed-loop steamcooling circuits, although the latter is the most likely application. Itcould also be applied to conventional air-cooled buckets.

Reduction of tip loss improves component efficiency and thereby improvesthe efficiency and the power output of the gas turbine. This in turnreduces the amount of pollutants emitted into the environment for agiven amount of power production, and improves the operating economicsof the gas turbine power plant.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A stage-one, steam-cooled bucket for a steam turbine comprising anairfoil portion incorporating a closed-loop steam cooling circuit andhaving a radially outer tip cap secured thereto, said radially outer tipcap covered with a thermal barrier coating, and wherein said thermalbarrier coating is provided with a cavity defined by an area of reducedcoating thickness in said thermal barrier coating extending along atleast a center portion of said radially outer tip cap, wherein saidcavity forms a ridge along only said suction side of said airfoilportion.
 2. The bucket of claim 1 wherein said cavity has a depth of30-60 mils.
 3. A stage-one, steam-cooled bucket for a steam turbinecomprising an airfoil portion incorporating a closed-loop steam coolingcircuit and having a radially outer tip cap secured thereto, saidradially outer tip cap covered with a thermal barrier coating, andwherein said thermal barrier coating is provided with a cavity definedby an area of reduced coating thickness in said thermal barrier coatingextending along at least a center portion of said radially outer tipcap, wherein said cavity forms a ridge along only said pressure side ofsaid airfoil portion.
 4. The bucket of claim 3 wherein said cavity has adepth of 30-60 mils.
 5. A method of reducing tip leakage loss at aradially outer tip of a stage-one, steam-cooled turbine bucketcomprising: (a) providing a stage-one, steam turbine bucketincorporating a closed-loop steam cooling circuit; (b) coveringsubstantially entirely the radially outer tip of the bucket with athermal barrier coating of first predetermined thickness; (c)resurfacing by machining or grinding all of the thermal barrier coatingto reduce said first predetermined thickness and including resurfacingthe thermal barrier coating in selected portions of said thermal barriercoating to thereby define a ridge in said coating, extending along onlya pressure side of said bucket and along substantially the entire lengthof said radially outer tip.
 6. The method of claim 5 wherein said cavityhas a depth of between 6 and 30 mils.
 7. The bucket of claim 5 whereinsaid coating has a minimum thickness of about 30 mils.
 8. A method ofreducing tip leakage loss at a radially outer tip of a stage-one,steam-cooled turbine bucket comprising: (a) providing a stage-one, steamturbine bucket incorporating a closed-loop steam cooling circuit; (b)covering substantially entirely the radially outer tip of the bucketwith a thermal barrier coating of first predetermined thickness; (c)resurfacing by machining or grinding all of the thermal barrier coatingto reduce said first predetermined thickness and including resurfacingthe thermal barrier coating in selected portions of said thermal barriercoating to thereby define a ridge in said coating, extending alongsubstantially the entire length of said radially outer tip; and whereinsaid ridge extends along only a suction side of said bucket.
 9. Themethod of claim 8 wherein said cavity has a depth of between 6 and 30mils.
 10. The bucket of claim 8 wherein said coating has a minimumthickness of about 30 mils.